Whether or not dogs are opportunists or not is irrelevant.
Yes they will eat anything but then so will any species if it is hungry enough AND if the source of the food is processed enough (cows and sheep eating chickens etc)? Ring any bells.
Dentition alone is not enough of a sign of a carnivore (bears and pandas are proof of that) you have to take the whole physiognomy into account.
Dogs are carnivores; full stop; yes they are omnivorous which means they CAN and DO eat other things than meat and, unlike cats that are OBLIGATE carnivores, can get some nutrition from non meat sources.
Does not change the FACT that they are carnivores.
Of COURSE feeding meat ONLY will not be healthy for dogs, that is why nature designed them to eat BONES. The prey animal is made up of protein, fat, fibre, and vegetable matter thereby providing a wholesome balanced diet along with micronutrients that they consume accidently or incidentally along the way such as windfall fruit, faeces, bark etc etc etc.
If you feed raw correctly it is based around the prey animal. If you look at the protein, fat, vitamin, mineral needs of a dog ALL can be provided for using the prey animal as a model with NO need for any form of cereals!
Whether you choose to feed cereals or not is a personal choice.
The discussion here centres around a) is the dog a carnivore - yes it is and b) do dog have any nutritional requirement for carbohydrates - no they do not the old Waltham site had a whole page devoted to this very subject; a company that sells commercial food made mainly from carbohydrates stated on their site that dogs have no nutritional need for carbs!!!!!
It is not available now, I wonder why....................
Just as horses, sheep and cows are herbivores; this is not an opinion but a fact